Feedback circuit for output control in a semiconductor X-ray detector

ABSTRACT

An X-ray detector using a semiconductor detector, most preferably a Silicon Drift Detector, utilizes a field effect transistor or other voltage-controlled resistance to generate an output voltage proportional to its input charge (which is generated by the X-ray photons incident on the semiconductor detector). To keep the charge (and thus the output voltage) to an acceptable range—one wherein the relationship between output voltage and input charge is substantially proportional—a feedback circuit is provided between the output and input terminals, wherein the charge on the input terminal is depleted when the output voltage begins leaving the desired range. Preferably, this is done by a comparator which monitors the output voltage, and provides a reset signal to the input terminal when it begins moving out of range. Alternatively or additionally, the reset signal may be a pulse supplied to the input terminal from a pulse generator activated by the comparator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/495,867, filed Jul. 28, 2006 now U.S. Pat. No. 7,339,175,entitled “X-Ray Detector”, which is incorporated by reference herein.

FIELD OF THE INVENTION

This document concerns an invention relating generally to X-raydetectors, and more specifically to X-ray detectors used inmicroanalysis and X-ray fluorescence measurement (e.g., in electronmicroscopes and X-ray spectrometers).

BACKGROUND OF THE INVENTION

X-ray detectors are used in electron microscopes (e.g., scanningelectron microscopes and tunneling electron microscopes), X-rayspectrometers (e.g., X-ray fluorescence spectrometers/energy-dispersiveX-ray spectrometers), and other instruments to analyze the compositionand properties of materials. An illumination source (such as an electronbeam or X-ray source) is directed at a sample to be analyzed, resultingin emission of X-rays from the sample wherein the X-rays (X-ray photons)have energies which are characteristic of the atoms of the sample fromwhich they were emitted. Thus, the counts of the emitted X-rays(photons) and their energies can indicate the composition of the sample.

Traditionally, X-ray detectors utilized lithium-doped silicon detector(Si(Li)) detectors. These detectors are semiconductor detectors whichgenerate a charge upon receiving X-rays, and thereby allow counting ofX-ray photons and measurement of their energies. In an arrangement usedby Thermo Electron Scientific Instruments (Madison, Wis., USA), theSi(Li) detector output was preamplified using a FET, i.e., a fieldeffect transistor, and the signal at the FET output (drain) was thenprovided to a cascode amplifier and pulse processor. To keep the FET inits linear range, feedback was used from the FET output (drain) to theFET input (gate). While this arrangement worked well, itdisadvantageously had a response which is rather slow by presentstandards—it could not accommodate X-ray photon counts of greater thanapproximately 60,000 counts per second—and additionally it requiredcooling to cryogenic temperatures for greatest accuracy.

In recent years, more advanced semiconductor detectors, such as SiliconDrift Detectors (SDDs), have become available, and these offer thepossibility of far greater count rate measurement (a million counts persecond or more) with lesser temperature control burdens (see, e.g.,Iwanczyk et al., High Throughput High Resolution VorteX™ Detector forX-Ray Diffraction, IEEE Transactions On Nuclear Science, Vol. 50, No. 6,December 2003). Unfortunately, SDD's carry their own drawbacks, inparticular the problem of varying detector response with x-rayillumination level: the SDDs, which often have an integrated FETprovided for signal amplification purposes, have a gain which changeswith voltage (Hansen et al., Dynamic Behavior of the Charge-to-VoltageConversion in Si-Drift Detectors With Integrated JFETs, IEEETransactions On Nuclear Science, Vol. 50, No. 5, October 2003). Thisresults in variations in the measurements of detected X-rays, and inturn difficulties in interpreting their significance. For example, for amanganese K-alpha X-ray detected by an SDD, as the measured photon countrate goes from 10,000 counts per second (10 kcps) to 100 kcps, itsmeasured energy can shift by 50-100 eV (a phenomenon known as peakshift), and resolution (the peak width on an intensity/counts vs.measured energy scale) can degrade by 10-30 eV. (Fiorini, C, A chargesensitive preamplifier for high peak stability in spectroscopicmeasurements at high counting rates, IEEE Transactions On NuclearScience, Vol. 52, No. 5, October 2005; Niculae, A.; Optimized ReadoutMethods of Silicon Drift Detectors for High Resolution X-RaySpectroscopy, 2005 Denver X-ray Conference, Colorado Springs, 5 Aug.2005). As noted by Fiorini, feedback to the FET output (drain) can helpreduce peak shift (shift in the peak on an intensity/counts vs. measuredenergy scale), but it does not solve the problem of resolutiondegradation. Peak shift can also be reduced by using feedback to controlthe source current, as described in European Patent Application EP 1 650871 A1.

Another method for reducing degradation at high count rates is to use a“pulsed reset,” wherein the FET input (gate) is reset through a diodeafter a predetermined period to some datum range or value of voltage(s).This method, which is discussed in the Niculae reference above, is incontrast to “continuous reset” methods, which use leakage current in theFET to discharge the gate (Bertuccio et al., Silicon drift detector withintegrated p-JFET for continuous discharge of collected electronsthrough the gate junction, Nucl. Instr. & meth. A 377 (1996)).

Despite all of the foregoing methods, additional methods would beuseful, since most of the foregoing methods do not significantly assistin reducing one or more of peak shift and resolution degradation, and inany event further reduction in peak shift and resolution degradationwould be useful.

SUMMARY OF THE INVENTION

The invention, which is defined by the claims set forth at the end ofthis document, is directed to X-ray detectors which at least partiallyalleviate the aforementioned problems. A basic understanding of some ofthe features of preferred versions of the detectors can be attained froma review of the following brief summary of the invention, with moredetails being provided elsewhere in this document. To assist in thereader's understanding, the following review makes reference to theaccompanying drawings (which are briefly reviewed in the “BriefDescription of the Drawings” section following this Summary section ofthis document).

Referring to FIG. 1 for a schematic depiction of a basic exemplaryversion of the invention, a sample 10 (e.g., a sample for microanalysis)is “illuminated” by an electron beam, X-ray beam, or other beam 12suitable to cause X-ray emissions 14 from the sample. The X-ray detector100, which may be considered to include the remaining elements shown inFIG. 1, then receives the X-rays 14 and provides an output signalV_(OUT) corresponding to the timing and energy of the constituentphotons of the X-rays 14. The X-rays 14 are received by a semiconductordetector 102, which preferably takes the form of an SDD (Silicon DriftDetector) and which is schematically depicted as a power source sincethe impinging photons of the X-rays 14 will induce corresponding chargesin the semiconductor detector 102.

A voltage-controlled resistance 104 is then provided, and has an inputterminal 106 (connected in communication with the semiconductor detector102), an output terminal 108, and a control voltage supply terminal 110,wherein the output terminal 108 is biased by the control voltageterminal 110 (which is preferably supplied with an at leastsubstantially constant voltage supply V_(DD)) to have a voltage at leastsubstantially proportional to the voltage at the input terminal 106. Thevoltage-controlled resistance 104 preferably takes the form of a fieldeffect transistor (FET) operating in its ohmic/triode range, with gate(FET input) 106, source (FET output) 108, and drain (voltage supply)110, and thus the remainder of this discussion will assume a FET is usedas the resistance 104. A current source 112 is usefully provided toassist in biasing the FET 104 at the gate 106. As a result of thisarrangement, the voltage at the source/output terminal 108 varies inaccordance with the time and energy of the X-ray photons 14 incident onthe semiconductor detector 102.

Since the charge at the gate 106 may over time build to such an extentthat the FET 104 will saturate (i.e., no longer operate as avoltage-controlled resistance), a feedback circuit 114 is then usefullyprovided between the source/output terminal 108 and the gate/inputterminal 106. The feedback circuit 114 includes a reset control circuit(depicted generically in FIG. 1 at 116) which supplies a reset voltagesignal to the gate/input terminal 106 (i.e., a voltage discharging thegate/input terminal 106 and “resetting” the FET 104) if the voltage ofthe source/output terminal 108 attains a threshold magnitude.Preferably, the feedback circuit 114 receives the source/output terminal108 voltage through a buffer 118, which is depicted as an invertingamplifier. The feedback circuit 114 additionally includes a diode 120having its cathode side 122 at the gate/input terminal 106, therebyenforcing one-way current flow through the feedback circuit 114 from thesource/output terminal 108 to the gate/input terminal 106, but not fromthe gate/input terminal 106 to the source/output terminal 108. Owing tothe feedback circuit 114, and more particularly the reset controlcircuit 116, the FET 104 (and more specifically the voltage of thesource/output terminal 108) is constrained to operate within apredefined range—a range having substantially linear input/outputcharacteristics—such that the voltage at the source/output terminal 108more accurately reflects the charge at the gate/input terminal 106, andthus the timing and energy of received X-ray photons 14 at thesemiconductor detector 102. Note that the phantom/dashed-line block 124,which includes the semiconductor detector 102, FET 104, and diode 120,is depicted because the FET 104 and diode 120 can be provided on boardthe chip for the semiconductor detector 102, and thus these componentscan be conveniently provided as a unitary assembly.

FIG. 2 then depicts the X-ray detector of FIG. 1 with an exemplaryversion of the reset control circuit. Here, as with the X-ray detector100, the X-ray detector 200 includes a semiconductor detector 202; avoltage-controlled resistance (FET) 204 having an input terminal (gate)206, an output terminal (source) 208 providing an output voltage V_(OUT)through a buffer 218, and a voltage supply terminal (drain) 210; and afeedback circuit 214 with diode 220. The reset control circuit 116 ofFIG. 1 is here depicted as including a comparator 226 (which will bereferred to as a “reset comparator” for reasons discussed below), apulse generator 228, and a capacitor 230. The reset comparator 226receives the voltage V_(OUT) from the source 208 (with V_(OUT) actuallybeing proportional to the inverted source 208 voltage, if the buffer 218is an inverting amplifier), and causes a reset (discharging) signal tobe sent to the gate 206 if V_(OUT) attains a threshold magnitude. Thisis done by the reset comparator 226 comparing V_(OUT) to an at leastsubstantially constant threshold voltage V_(THR), and providing anoutput signal to the pulse generator 228 when V_(OUT) is greater thanthe threshold voltage V_(THR). In response, the pulse generator 228outputs a discrete reset voltage signal pulse to the gate 206 whichdischarges it, and thereby resets V_(OUT) into an acceptable range. Thecapacitor 230 is useful to communicate the reset voltage signal pulse tothe gate 206 while blocking sustained/DC signals.

FIG. 3 then illustrates the X-ray detector of FIG. 1 with anotherexemplary version of the reset control circuit. Here the X-ray detector300 includes components as in the X-ray detector 200, but anothercomparator 332—which will be referred to as a protection comparator forreasons that will shortly be apparent—is also interposed between theoutput terminal (source) 308 (via the buffer 318) and the input terminal(gate) 306, and in parallel with the pulse generator 328 and resetcomparator 326. As in the X-ray detector 200, if the voltage of thesource 308 (as reflected by V_(OUT)) attains a threshold magnitudeV_(THR1), the reset comparator 326 causes the pulse generator 328 toissue a reset voltage signal pulse to discharge the gate 306, andthereby reset the voltage of the source 308 (and thus V_(OUT)) to adesired level. However, in some cases a single brief reset voltagesignal pulse may be insufficient to discharge the gate 306. Subsequentpulses from the pulse generator 328 may then be sufficient, but it wouldbe preferable to more rapidly discharge the gate 306 without the need toawait subsequent pulses. Thus, the protection comparator 332 is providedso that if the voltage of the output terminal (as reflected by V_(OUT))attains a threshold magnitude V_(THR2) (which may be set equal toV_(THR1)), the protection comparator 332 supplies a sustained resetvoltage signal to the gate 306, one which remains so long as V_(OUT) hasa magnitude exceeding a predetermined voltage level. Thus, in the X-raydetector 300, the reset comparator 326 supplies at least a portion ofthe reset voltage signal to the gate 306 if the output voltage V_(OUT)attains a first threshold magnitude V_(THR1), and the protectioncomparator 332 supplies at least a portion of the reset voltage signalto the gate 306 if the output voltage V_(OUT) attains a second thresholdmagnitude V_(THR2) (wherein V_(THR2) may equal to V_(THR1)).

FIG. 4 then illustrates yet another exemplary X-ray detector 400,wherein the X-ray detector 100 (and/or 200/300) is provided in acharge-sensitive amplifier arrangement. A capacitance C_(FB) is providedin parallel with the feedback circuit 414, which includes the resetcontrol circuit 416 and diode 420. It is notable that the capacitanceC_(FB) need not be provided by a conventional capacitor, and it couldinstead (or also) be merely provided as the stray capacitance of thesemiconductor detector 402 (e.g., the stray capacitance of the innerguard ring of an SDD semiconductor detector 402). Such a straycapacitance would more accurately be depicted within the detector block424, rather than outside it. Here, the buffer 418 should take the formof an inverting amplifier with nonunity gain, whereas the buffers118/218/318 of the detectors 100/200/300 need not be inverting and mayhave unity gain (though use of a nonunity-gain inverting amplifier ispreferred in all detectors).

FIG. 5 illustrates still another exemplary X-ray detector 500, one whichmodifies the X-ray detector 200 of FIG. 2 to incorporate additionaluseful features. Here the reset comparator 526—depicted as anoperational amplifier with hysteresis (i.e., a lag in output)—has anadditional input V_(OFF), as well as inputs V_(OUT) and V_(THR). InputsV_(OUT) and V_(THR) operate as in the detector 200: when the resetcomparator 526 receives a voltage V_(OUT) from the source 508 which isgreater than a threshold voltage V_(THR), a reset (discharging) signalis sent to the gate 506 (preferably via a pulse generator 528, similarlyto the detector 200). However, since a single pulse or other resetsignal may be insufficient to fully discharge the gate 506, in this casethe reset comparator 526 exhibits hysteresis, and continues to deliverthe reset signal (and thereby activate the pulse generator 528, ifpresent) even if V_(OUT) thereafter drops below the threshold voltageV_(THR). If V_(OUT) subsequently drops to a turn-off magnitudeV_(OFF)—V_(OFF) being less than V_(THR)—the reset comparator 526 willthen halt the reset signal until V_(OUT) once again reaches thethreshold voltage V_(THR). This arrangement advantageously maintains thereset signal (e.g., continues to deliver discharging pulses) at the gate506 until V_(OUT) drops to a desired level (which is in part determinedby V_(OFF)).

Further features and advantages of the invention will be apparent fromthe remainder of this document in conjunction with the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram depicting a first X-ray detector100 exemplifying the invention.

FIG. 2 is a schematic circuit diagram depicting an X-ray detector 200generally corresponding to the X-ray detector 100 of FIG. 1, but whereinthe reset control circuit 116 is provided by a reset comparator 226, apulse generator 228, and a capacitor 230.

FIG. 3 is a schematic circuit diagram depicting an X-ray detector 300which also generally corresponds to the X-ray detector 100 of FIG. 1,but wherein the reset control circuit 116 is provided by a protectioncomparator 332 in parallel with a reset comparator 326, pulse generator328, and capacitor 330.

FIG. 4 is a schematic circuit diagram depicting an X-ray detector 400wherein the X-ray detector 100 is adapted into a charge-sensitiveamplifier arrangement, with a capacitor C_(FB) in parallel with thefeedback circuit 414.

FIG. 5 is a schematic circuit diagram depicting an X-ray detector 500which resembles the X-ray detector 200 of FIG. 2, but wherein the resetcomparator 526 supplies a reset voltage signal to the gate 506 ifV_(OUT) reaches or exceeds a threshold magnitude V_(THR), and the resetvoltage signal thereafter remains on until V_(OUT) drops to or below aturn-off magnitude V_(OFF).

DETAILED DESCRIPTION OF PREFERRED VERSIONS OF THE INVENTION

Following is a list of exemplary components that may be used in theforegoing detectors, though it should be understood that any othersuitable components may be used.

The semiconductor detectors 102/202/302/402/502 (and more generally thedetector chips/blocks 124/224/324/424/524) may take the form ofPSD-10-130, PSD-30-140, or SD3-10-128 Silicon Drift Detector modulesfrom PNSensor GmbH (Munich, Germany), which usefully have an onboardFET.

The amplifiers/buffers 118/218/318/518 may take the form of the AD829operational amplifier (Analog Devices, Norwood, Mass., USA) operating innet unity gain. However, this is not preferred for the amplifier 418,which has nonunity gain for optimal operation. Thus, the amplifiers118/218/318/418/518 may take the form of a two-stage amplifier using theforegoing AD829 operational amplifier, e.g., with the first stageamplifier providing a non-inverting voltage gain of 16, and the secondstage amplifier providing an inverting voltage gain of 4 (and with thestages being frequency-compensated for fast response with maximum signalto noise).

In the X-ray detectors 200 and 300, the reset comparators 226/326 andprotect comparator 332 may utilize the LM311 high-speed comparator(National Semiconductor, Santa Clara, Calif., USA) for sensing thevoltage level of the amplifiers/buffers 118/218/318/418. The pulsegenerator 228/328 may utilize the CD4538 monostable multivibrator(National Semiconductor). Alternatively, the pulse generator 228/328 maybe generated from a current source using a transistor (e.g., theMMBT5087 PNP transistor from Motorola, Schaumburg, Ill., USA), anarrangement which is preferred when the protect comparator 332 is used.

In the X-ray detector 500, the reset comparator 526 can be built from acomparator having resistive feedback, as described in Chapter 4 of theSecond Edition of Horowitz & Hill's The Art of Electronics (CambridgeUniversity Press, 1990). A high-speed comparator, e.g., the MAX912 orMAX913 comparators (Maxim Integrated Products, Sunnyvale, Calif., USA),can allow quick response and can be used to operate an oscillatorcircuit (the pulse generator 528) as described in Chapter 5 of Horowitz& Hill. The pulse width of the pulse generator 528 could be controlledby using a monostable multivibrator, e.g., the CD4538BC dual precisionmonostable (Fairchild Semiconductor Corporation, Portland, Me., USA).

Detectors in accordance with the invention can readily accomplishminimal peak shift and resolution degradation at extremely high countrates, with the components discussed above providing peak shift of lessthan 2 eV in the 10-100 kcps count range. Further, the resolutiondegradation (increase in Full Width at Half Maximum or FWHM) isstatistically insignificant (repeatability of measurements is ±1 eV). Acomparison of these results with those reported in the references notedat the outset of this document shows that the invention appears to haveperformance exceeding all prior arrangements. In addition, the inventionis believed superior to prior “resetting” methods insofar as resets areonly imposed if/when the source voltage (and thus V_(OUT)) reaches theend of its acceptable range. Since the gate cannot collect charge fromincident x-rays during resetting—i.e., since counts are “lost” duringresetting—the invention minimizes reset events, and thus minimizes lostcounts.

The detector 500 of FIG. 5 is particularly preferred because it has beenfound to allow input count rates in excess of 1.5 Mcps, usinghigh-resolution detectors 502 (and detector chips/blocks 524) availableas of 2007. Where a single reset pulse is sufficient to bring V_(OUT) tothe desired level, only a single reset pulse is delivered, but if asingle pulse is insufficient, pulses are delivered until V_(OUT) attainsthe desired level. It should be understood that within the string ofpulses generated by the pulse generator 528, pulses may vary—forexample, successive pulses may differ in amplitude, width, shape, and/orduty cycle—until V_(OUT) reaches the desired level.

It should also be understood that the detector arrangements describedabove are merely exemplary, and detectors in accordance with theinvention can include different components in different arrangements. Asan example, the X-ray detectors 200 and 300 of FIGS. 2 and 3 depict avoltage V_(RD) also being supplied to the feedback circuits 214 and 314to reverse-bias the diodes 220 and 320. This arrangement is useful toassist in noise reduction, but is not mandatory. Further, other resetcontrol circuit arrangements are possible apart from those discussedabove. As one example, a time delay can be incorporated into thefeedback circuit 214/314 so that the reset pulse issued by the pulsegenerator 228/328, and/or the reset signal issued by the protectioncomparator 332, is supplied to the gate after a delay, thereby allowingadequate time to register the count (X-ray photon impingement) givingrise to the reset pulse/signal. Additionally, other voltage-controlledresistances may be used in place of a FET 204; other buffers could beused in place of inverting amplifiers 118/218/318/418; and so forth.

In summary, the invention is not intended to be limited to the exemplaryversions described above, but rather is intended to be limited only bythe claims set out below. Thus, the invention encompasses all differentversions that fall literally or equivalently within the scope of theseclaims.

1. An x-ray detector including: a) a semiconductor detector; b) avoltage-controlled resistance having: (1) a voltage supply, (2) an inputterminal in communication with the semiconductor detector, and (3) anoutput terminal, wherein the output terminal is biased by the voltagesupply to have a voltage dependent on the voltage at the input terminal;c) a feedback circuit between the output terminal and the inputterminal, wherein the feedback circuit includes a comparator configuredto supply a reset voltage signal to the input terminal if the voltage ofthe output terminal attains a threshold magnitude, and wherein thecomparator is further configured to continue to supply the reset voltagesignal to the input terminal until the voltage of the output terminalattains a turn-off magnitude, the turn-off magnitude being less than thethreshold magnitude.
 2. The x-ray detector of claim 1 wherein thesemiconductor detector is a silicon drift detector (SDD).
 3. The x-raydetector of claim 1 wherein the voltage-controlled resistance includes afield effect transistor having: a) a drain in communication with thevoltage supply; b) a gate in connection with the input terminal, and c)a source in connection with the output terminal.
 4. The x-ray detectorof claim 1 wherein the feedback circuit allows current flow from theoutput terminal to the input terminal, but not from the input terminalto the output terminal.
 5. The x-ray detector of claim 1 wherein thefeedback circuit includes a diode having a cathode in communication withthe input terminal.
 6. The x-ray detector of claim 1 wherein thefeedback circuit further includes a pulse generator, wherein the pulsegenerator is configured to be activated by the comparator so as to sendone or more reset voltage signal pulses to the input terminal if thevoltage of the output terminal attains the threshold magnitude.
 7. Thex-ray detector of claim 6 further including a capacitor interposedbetween the pulse generator and the input terminal.
 8. The x-raydetector of claim 6 wherein the comparator comprises a reset comparator.9. The x-ray detector of claim 8 wherein the reset comparator isconfigured to continually trigger the pulse generator so as to supplythe one or more reset voltage signal pulses to the input terminal untilthe voltage of the output terminal attains a turn-off magnitude, theturn-off magnitude being less than the threshold magnitude.
 10. Thex-ray detector of claim 8 further including a protection comparatorinterposed between the output terminal and the input terminal, and inparallel with the pulse generator and the reset comparator, wherein theprotection comparator is configured to supply a sustained reset voltagesignal to the input terminal if the voltage of the output terminalattains a threshold magnitude.
 11. The x-ray detector of claim 1 whereinthe feedback circuit includes: a second comparator interposed betweenthe output terminal and the input terminal, and in parallel with thecomparator additionally configured to supply at least a portion of thereset voltage signal to the input terminal if the voltage of the outputterminal attains a first threshold magnitude, and wherein the secondcomparator is configured to supply at least a portion of the resetvoltage signal to the input terminal if the voltage of the outputterminal attains a second threshold magnitude.
 12. An x-ray detectorincluding: a) a semiconductor detector; b) a FET having: (1) a gate incommunication with the semiconductor detector; (2) a drain maintained atan at least substantially constant drain voltage; and (3) a source, c) afeedback circuit between the source and the gate, wherein the feedbackcircuit includes a comparator configured to supply a voltage signal tothe gate if the source voltage attains a threshold magnitude, andwherein the comparator is further configured to supply the voltagesignal to the gate until the source voltage drops below a turn-offmagnitude which is less than the threshold magnitude.
 13. The x-raydetector of claim 12 further including a pulse generator, wherein thecomparator is configured to activate the pulse generator so as to supplythe voltage signal to the gate if the source voltage attains thethreshold magnitude.
 14. The x-ray detector of claim 12 wherein thefeedback circuit includes a capacitor between the comparator and thegate.
 15. The x-ray detector of claim 14 further including a secondcomparator in parallel with the capacitor, wherein the second comparatoris configured to supply a protect voltage signal to the gate if thesource voltage attains a second threshold magnitude.
 16. The x-raydetector of claim 15 further including a pulse generator in series withthe capacitor and in parallel with the second comparator.
 17. An x-raydetector including: a) a semiconductor detector; b) a PET having: (1) agate in communication with the semiconductor detector; (2) a drainmaintained at an at least substantially constant drain voltage; and (3)a source, c) a feedback circuit between the source and the gate, whereinthe feedback circuit includes: (1) a comparator receiving: (a) an atleast substantially constant threshold voltage, and (b) a voltageproportional to the voltage of the source, the comparator configured toprovide an output signal when the proportional voltage is greater thanthe threshold voltage; and (2) a pulse generator in communication withthe comparator, wherein the pulse generator is configured to output oneor more reset voltage signal pulses upon receiving the comparator outputsignal.